1. Field of the Invention
The present invention relates to a vacuum plant, in particular for secondary metallurgy, having at least one steam ejector which is in communication with a closed container within which a metallurgical vessel which is to be subjected to a vacuum can be brought. The vacuum plant further has a second steam ejector connected in series with the first steam ejector via an intermediate condenser having a cooling-water feed line and a condensate discharge line. A final condenser and a suction pump are connected in series to the second steam ejector.
2. Description of the Prior Art
Vacuum plants which are to subject large volumes in the shortest possible time to a pressure of about 0.5 mbar are presently used in the metallurgical industry. For the drawing-off of air, steam ejectors are employed, at least two of which--and frequently three--depending on the design of the plant, are connected in series without an intermediate condenser. These steam ejectors have centrally arranged nozzles which are operated with steam.
The working agent, steam, in this connection drives air or gas in a known manner. While pure air or gases are still being drawn-in in the first stage, a mixture of steam and air or gas which must be driven by the steam working agent in the second stage, leaves the first stage. In each further stage, the proportion of steam in the driving fluid increases, with the result that the use of the driving agent must be increased in each following stage. In this way, the overall efficiency of the vacuum installation decreases. To be sure, the efficiency of the individual ejector stages is about equal and the increasing amount of steam in each stage causes a substantial impairment of the efficiency of the plant.
In order to reduce the steam content of the driven fluid, condensers are employed. As cooling water, untreated industrial water is generally employed. With this cooling water, it is not possible to remove the desired, once-established amount of moisture from the working fluid at low pressures or to condense the steam, since this cooling water has at least ambient temperature and the pressure in the condenser can in no case be brought below the vapor pressure corresponding to the temperature of the cooling water.